Optical ring networks, in which a plurality of nodes are connected by optical fibers to form a ring, have been put into practical use in optical network configurations. For example, a metropolitan area network (MAN) includes a plurality of optical nodes connected by optical fibers to form a ring in a city or a local area. In that configuration, each optical node is often provided with an optical add drop multiplexer (OADM). Also, a plurality of optical ring networks may sometimes be connected to each other in order to expand the communication area or to increase the communication capacity. In such a case, an optical network interconnect device is used to interconnect two or more optical ring networks. An optical network interconnect device may also be called a hub node.
Meanwhile, wavelength division multiplexing (WDM) technology is used in practice for optical communications. WDM enables large capacity communications because WDM utilizes plural wavelengths to transmit plural optical signals through an optical fiber. There is also a method under development that makes wavelength channel spacing narrower in order to further increase the transmission capacity of WDM networks. For example, in many current metropolitan area networks, the wavelength channel spacing of a WDM optical signal is 100 GHz. However, it is expected that many WDM systems will provides 50 GHz-spaced WDM channels.
Accordingly, in the foreseeable future, 100 GHz-spaced WDM networks and 50 GHz-spaced WDM networks coexist. Thus, it can be thought that there is demand for a configuration that interconnects a 100 GHz-spaced WDM network and a 50 GHz-spaced WDM network.
As a related technique, an optical network interconnect device that connects nodes of the first and second optical networks transmitting optical wavelength division multiplexed signals is proposed. An optical network interconnect device converts the signal rate and/or the optical signal wavelength of an optical signal having a particular wavelength that has been transmitted from a node of the first optical network, and adds the resultant signal to a wavelength division multiplexed signal of the second optical network. Further, this optical network interconnect device includes an photo-electric converter to perform photo-electric conversion on light transmitted from a node of the first optical network, an separator to separate an electric signal obtained by the photo-electric converter into plural electric signals, a plurality of optical modulators to optically modulate particular wavelengths of individual electric signals obtained by the separator, and an optical coupler to multiplex lights output from the plurality of optical modulators (Japanese Laid-open Patent Publication No. 2001-36479, for example).
Related techniques are also described in Japanese Laid-open Patent Publication No. 2006-86920, Japanese Laid-open Patent Publication No. 2004-297228, and International Publication Pamphlet No. WO2005/096534.
FIG. 1 illustrates an example of a configuration to connect WDM networks that with different wavelength spacing. In the example illustrated in FIG. 1, a WDM network 1 transmits 50 GHz-spaced WDM optical signal. A WDM network 2 transmits 100 GHz-spaced WDM optical signal.
The WDM network 1 has a plurality of node devices 11 through 14 connected by optical fibers to form a ring. In this example, each of the node devices 11 through 14 is a reconfigurable OADM (ROADM) to process 50 GHz-spaced WDM optical signal. The WDM network 2 has a plurality of node devices 21 through 24 connected by optical fibers to form a ring. In this example, each of the node devices 21 through 24 is a reconfigurable OADM to process 100 GHz-spaced WDM optical signal.
An optical network interconnect device (HUB node) 3 relays optical signals between the WDM network 1 and the WDM network 2. In this example, the optical network interconnect device 3 includes the node devices 11 and 21 belonging to the WDM networks 1 and 2, respectively.
In the WDM network 1, respective channels of a WDM optical signal are allocated at wavelengths λ1, λ2, λ3, λ4 . . . as illustrated in FIG. 2A. The spaces between wavelengths λ1, λ2, λ3, λ4 . . . are 50 GHz. In the WDM network 2, respective channels of a WDM optical signal are allocated at wavelengths λ2, λ4, λ6 . . . . The spaces between λ2, λ4, λ6 . . . are 100 GHz. Each of the node devices 21 through 24 in the WDM network 2 provides transmission bands with a spacing of 100 GHz in order to transmit optical signals having wavelengths λ2, λ4, λ6 . . . as illustrated in FIG. 2B.
Operations of transferring an optical signal of the WDM network 1 to the WDM network 2 in FIG. 1 are discussed. The optical network interconnect device 3 guides, to the node device 21, the 50 GHz-spaced WDM optical signal that is dropped by the node device 11. In this configuration, it is assumed that the node device 21 selects, for example, channels ch2 and ch4 from the WDM optical signal dropped by the node device 11, and adds the selected channels to the WDM network 2. In such a case, the node device 21 provides transmission bands A and B as illustrated in FIG. 2C. The center wavelengths of transmission bands A and B are λ2 and λ4, respectively. Then, optical signals in channels ch2 and ch4 are added to the WDM network 2.
However, the transmission bands provided by the respective node devices 21 through 24 of the WDM network 2 is designed to select/remove an optical channel in the 100 GHz-spaced WDM optical signal. In other words, the width of the transmission bands provided by the node devices 21 through 24 is set to be greater than that of transmission bands to select/remove an optical channel in the 50 GHz-spaced WDM optical signal. Because of this, when the 50 GHz-spaced WDM optical signal is guided from the node device 11 to the node device 21, unnecessary wavelength components enter the WDM network 2.
For example, when the node device 21 provides the transmission band A in order to select channel ch2, part of spectrums of channels ch1 and ch3 passes through the transmission band A. In such a case, signal components of channels ch1 and ch3 that pass through the transmission band A cause crosstalk with optical signal in channel ch2, deteriorating the quality of channel ch2. In a similar manner, the quality of channel ch4 is deteriorated by crosstalk caused by channels ch3 and ch5 that have passed through the transmission band B.
This problem may be solved by, for example, providing a wavelength conversion and regeneration relay device between WDM networks (i.e., between the node devices 11 and 21). In such a case, the wavelength conversion and regeneration relay device demultiplexes WDM optical signal with respect to wavelength to generate a plurality of optical signals, and converts the respective optical signals into electric signals. Next, the wavelength conversion and regeneration relay device converts the respective electric signals into optical signals having specified wavelengths. Thereafter, the wavelength conversion and regeneration relay device multiplexes the optical signals to generate a WDM optical signal. However, this configuration requires a larger sized optical network interconnect device because it has an O/E converter and an E/O converter for each wavelength. Also, such an optical network interconnect device for is expensive.